1. Field of the Invention
This invention relates to adjustable waveguide short circuits and, in particular, to computer-controlled adjustable non-contacting waveguide short circuits. Accordingly, it is a general object of this invention to provide new and improved waveguide short circuits of such character.
2. General Background
An adjustable waveguide short circuit is desirable as a diagnostic tool for carrying out various measurements including determining the noise figure of a microwave amplifier.
As background, the noise figure for a microwave amplifier is given by the equation: EQU F=F.sub.min +4r.sub.n (.vertline..GAMMA..sub.s -.GAMMA..sub.0 .vertline..sup.2)/{(1-.vertline..GAMMA..sub.s .vertline..sup.2).vertline.1+.GAMMA..sub.0 .vertline..sup.2 }
where
r.sub.n =equivalent input noise resistance, PA1 F.sub.min =minimum noise figure, PA1 .GAMMA..sub.0 =source reflection coefficient for minimum noise figure, PA1 F=noise figure, PA1 .GAMMA..sub.s =source reflection coefficient for noise figure F.
In the foregoing equation, r.sub.n, F.sub.min, and .GAMMA..sub.0 are considered as device characteristics, and are generally determined by measurements. The relationship between F and .GAMMA.hd s is bilinear. For a given noise figure F, the solution for .GAMMA..sub.s lies on a circle. Thus, prior to circuit design, r.sub.n, F.sub.min, and .GAMMA..sub.0 should be determined. As .GAMMA..sub.0 is generally complex, there are four unknowns in the equation for F.
The unknowns can be determined by utilizing four input circuits having four known values of (.GAMMA..sub.s).sub.i, i=1 to 4, and measuring the noise figure F.sub.i, i=1 to 4, resulting from each of these (.GAMMA..sub.s).sub.i values. This results in four simultaneous equations with four unknowns, which can be solved to give the values of r.sub.n, F.sub.min and .GAMMA..sub.s. Generally, to improve accuracy, more than four values of .GAMMA..sub.s are used, and the unknowns are determined by regression technique.
Two alternative methods have been used to provide the required source (.GAMMA..sub.s).sub.i. One method uses a noise source followed by a tuner. Different values of (.GAMMA..sub.s).sub.i are obtained by different settings on the tuner. The second method utilizes an adjustable short circuit in series with a precision attenuator of known attenuation. This combination has been used to terminate the main arm of a directional coupler and a noise source is connected to the coupled arm. The magnitude of (.GAMMA..sub.s).sub.i is determined by the attenuator and the phase by the adjustable short circuit. Of these two methods, the second method is preferred and widely used in the determination of noise characteristics of low noise microwave amplifiers used in receivers.
With either method, the first step is to measure the source reflection coefficient, (.GAMMA..sub.s).sub.i, on an automatic network analyzer. The source is then connected to the amplifier under test, and the noise figure of the amplifier is measured. This procedure is repeated at least four times with a different .GAMMA..sub.s each time to obtain the necessary four equations in the four unknowns. Disadvantageously, this procedure is unreliable because in going from the analyzer to the amplifier under test the settings of the tuner or the short circuit may have changed inadvertently, causing subsequent measurements to be erroneous. The procedure is not reliably repeatable because it is very difficult to repeat exactly the settings of an adjustable tuner or a contacting type adjustable short circuit. Hence, previous settings and measured (.GAMMA..sub.s).sub.i cannot be reused. The prior art procedure has been tedious and time consuming, especially when more than one amplifier is to be characterized.